Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

The precise value of the mean neutron lifetime, τ_n, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties

Precision measurement of the neutron β-decay asymmetry

A new measurement of the neutron β-decay asymmetry A_0 has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to A_0=−0.11954(55)_(stat)(98)_(syst), corresponding to the ratio of axial-vector to vector coupling λ ≡ g_A/g_V = −1.2756(30)

Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

The precise value of the mean neutron lifetime, τ_n, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties

First direct constraints on Fierz interference in free-neutron β decay

Precision measurements of free-neutron β decay have been used to precisely constrain our understanding of the weak interaction. However, the neutron Fierz interference term b_n, which is particularly sensitive to beyond-standard-model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding b_n=0.067±0.005_(stat)^(+0.090)_(−0.061)_(sys), consistent with the standard model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the β spectrometer energy response

Performance of the Los Alamos National Laboratory spallation-driven solid-deuterium ultra-cold neutron source

In this paper, we describe the performance of the Los Alamos spallation-driven solid-deuterium ultra-cold neutron (UCN) source. Measurements of the cold neutron flux, the very low energy neutron production rate, and the UCN rates and density at the exit from the biological shield are presented and compared to Monte Carlo predictions. The cold neutron rates compare well with predictions from the Monte Carlo code MCNPX and the UCN rates agree with our custom UCN Monte Carlo code. The source is shown to perform as modeled. The maximum delivered UCN density at the exit from the biological shield is 52(9) UCN/cc with a solid deuterium volume of ∼1500 cm^3

Genetic Analysis of a Cryptic Contact Zone between Mitochondrial Clades of the Eastern Red-Backed Salamander, Plethodon cinereus

When evolutionarily divergent lineages adjoin their geographic ranges after a period of isolation, myriad outcomes can occur, from population anastomosis to the evolution of reproductive isolation by way of reinforcement. Hybrid zones represent natural experiments that may indicate whether lineages will maintain their evolutionary independence. Here, we report on a hybrid zone in the Eastern Red-Backed Salamander, Plethodon cinereus, a highly abundant and wide-ranging terrestrial salamander found in the northeastern United States and in southeastern Canada. An earlier study identified six distinct mitochondrial clades across the range of P. cinereus. Populations of two of these clades were as close as 9.6 km apart in Lorain County, Ohio, USA. To investigate the nature of this contact zone, we sampled 316 individuals from 16 sites along a 53-km transect, and analyzed 10 microsatellite loci and one mitochondrial locus. We found a clinal transition for mtDNA haplotypes. In contrast, most studies of terrestrial plethodontid salamanders commonly exhibit sharp boundaries between mtDNA clades. Microsatellite markers, however, revealed little differentiation and weak population structure, suggesting the nuclear cline, if it exists, lies outside of our sampling region. Explanations for the discordance between the mitochondrial DNA and our microsatellite data include lineage sorting, male-biased dispersal, or historical introgression of mtDNA, among other possibilities. We compare our results with other studies of introgression in terrestrial salamanders, and discuss the causes of mitonuclear discordance

A high-field adiabatic fast passage ultracold neutron spin flipper for the UCNA experiment

The UCNA collaboration is making a precision measurement of the β asymmetry (A) in free neutron decay using polarized ultracold neutrons (UCN). A critical component of this experiment is an adiabatic fast passage neutron spin flipper capable of efficient operation in ambient magnetic fields on the order of 1 T. The requirement that it operate in a high field necessitated the construction of a free neutron spin flipper based, for the first time, on a birdcage resonator. The design, construction, and initial testing of this spin flipper prior to its use in the first measurement of A with UCN during the 2007 run cycle of the Los Alamos Neutron Science Center's 800 MeV proton accelerator is detailed. These studies determined the flipping efficiency of the device, averaged over the UCN spectrum present at the location of the spin flipper, to be ϵ(overbar) = 0.9985(4)

Hybridization between the Woodland Salamanders Plethodon cinereus and P. electromorphus Is Not Widespread

A recent study reported widespread hybridization between the Eastern Red-backed Salamander (Plethodon cinereus) and the Northern Ravine Salamander (P. electromorphus) in northern Ohio. In this study, DNA sequence data were obtained from three nuclear loci and 20 single nucleotide polymorphisms (SNPs) were identified from the sequences. They found that 48 out of 90 individuals from 13 populations were hybrids, and in some localities every individual possessed an admixed genotype. As these results contradict our observations, and because levels of hybridization impact our interpretation of past and ongoing studies, we revisited the data. First we reanalyzed the original SNPs using STRUCTURE, then we repeated the analysis using haplotypes instead of SNPs. We found that K1⁄4 2 was best supported by both analyses, and they agree in recovering lower levels of hybridization than originally reported. For example, five populations in the original study identified as highly admixed or composed entirely of admixed genotypes we found to be pure P. cinereus or to lack evidence of extensive admixture. Similar results were obtained using NEWHYBRIDS and analyses based on gene trees. We conclude that while hybridization between P. cinereus and P. electromorphus occurs, it is much more restricted than originally reported

Minimal, superficial DNA damage in human skin from filtered far-ultraviolet C

Funding: This study was funded in part by MR/P012248/1 to R.P.H. from the Medical Research Council.Publisher PDFPeer reviewe

Designed Guanidinium-Rich Amphipathic Oligocarbonate Molecular Transporters Complex, Deliver and Release siRNA in Cells

The polyanionic nature of oligonucleotides and their enzymatic degradation present challenges for the use of siRNA in research and therapy; among the most notable of these is clinically relevant delivery into cells. To address this problem, we designed and synthesized the first members of a new class of guanidinium-rich amphipathic oligocarbonates that noncovalently complex, deliver, and release siRNA in cells, resulting in robust knockdown of target protein synthesis in vitro as determined using a dual-reporter system. The organocatalytic oligomerization used to synthesize these co-oligomers is step-economical and broadly tunable, affording an exceptionally quick strategy to explore chemical space for optimal siRNA delivery in varied applications. The speed and versatility of this approach and the biodegradability of the designed agents make this an attractive strategy for biological tool development, imaging, diagnostics, and therapeutic applications